Glass tubes of the aforesaid type, having lengths of up to several meters are required as starting material for manufacturing a wide range of glass elements, for example, for manufacturing bottles and containers, tubular covers for illuminants and for manufacturing illuminant tubes in lighting engineering. On the one hand, it is required that these glass tubes should be produced as cheaply as possible. On the other hand, in many applications, for example in lighting engineering, there is a demand for glass tubes having precisely finished inner profiles, for example, predetermined by the technical specifications in the respective application.
In the manufacture of glass tubes, a fundamental distinction is made between discontinuous and continuous manufacturing methods. Because of the fundamentally different process parameters in most cases, the principles applied in discontinuous manufacturing methods cannot or at least cannot easily be transferred to continuous manufacturing methods so that they cannot serve as a stimulus for the person skilled in the art to improve continuous manufacturing methods.
DE 497 649 discloses a method and apparatus for drawing glass tubes having round or non-round inner profiles using the so-called Danner method. In this method liquid glass is externally guided onto an inclined, rotating tubular body and drawn off as a tube at its lower end. In this method the inner profile of the drawn glass tube is defined by the outer profile of the tubular body at its lower end. In order to produce glass tubes having a non-round inner profile, the lower end of the tubular body can also have an outer contour which deviates from a circular shape. The inner profile of the glass tubes thus produced has comparatively high tolerances. The edge radii of glass tubes having non-round profiles are also comparatively large. Thus, this method frequently no longer meets the present requirements for the precision and tolerances of calibrated round or profiled glass tubes.
U.S. Pat. No. 2,009,793, the whole content of which is hereby incorporated, discloses the so-called Vello method for continuously manufacturing glass tubes having a circular inner profile. A substantially cone-shaped shaping body is arranged underneath an outlet opening of a melt channel, which serves as a receptacle for the molten glass, concentrically to the nutlet member of the melt channel. The emerging molten glass is drawn out by a drawing device through the annular gap between the shaping body and the outlet member so that a glass tube having a circular inner profile substantially predetermined by the outer contour of the shaping body is formed. Downstream from the shaping body the still deformable, tubular glass body is further stretched by means of a drawing device until this has finally cooled below the softening point. The inside diameter and the wall thickness of the glass tube is determined by the dimensions of the annular gap between the shaping body and the outlet member of the melt channel, by the temperature of the emerging molten glass, the temperature relationships downstream of the outlet opening and by the tensile force or drawing speed of the drawing device. It is relatively difficult to find suitable ranges of parameters for manufacturing glass tubes having different profiles and requires a great deal of experience which at least makes it expensive to completely automate the method. Re-equipping the melting furnace to manufacture glass tubes having different profiles is relatively expensive. Nevertheless, glass tubes manufactured by the Vello method have tolerances which increasingly are no longer suitable for the present applications.
FIG. 1a shows in a schematic sectional view an apparatus for manufacturing calibrated round or profiled glass tubes according to German patent DE 10 2004 018 148 B4 or corresponding US 2005/0227027 A1 of the applicant, the whole content of which is hereby incorporated. It comprises a shaping body 103 embodied as a drawing needle and a downstream profile forming body 104, which can be shifted in axial direction, for forming the inner profile of glass tube 105. While FIG. 1a shows the apparatus 100 in a position where the profile forming body 100 almost abuts against the shaping body, FIG. 1b shows this apparatus 100 in a position where the profile forming body 104 is arranged at a distance from the shaping body 103. In this apparatus 100 the shaping of the hot but still deformable drawing bulb 150 and the further shaping thereof to a calibrated round or profiled glass tube 105 can be accomplished in different zones, so that the process parameters of the drawing method can be set in a variable but controlled manner. The profile forming body can also be replaced by a different suitable profile forming body for varying the inner profile of the glass tube 105 to be manufactured.
For replacement of the profile forming body 104 by a different profile forming body firstly the flow of the glass melt 107 via the outlet ring 24 is stopped by lifting the shaping body 103 and closing the outlet orifice 112 of the melt channel 102. Then the profile forming body 104 is released from the rod 132 and a new profile forming body 104 is coupled with the rod 132 or the rod 132 is completely replaced together with the profile forming body 104. Subsequently, the outlet orifice 112 of the melt channel 102 is again opened to a sufficient extent by lowering the shaping body 103 and then the process is started again. The glass melt 106 is drawn over the shaping body 103 with a predetermined drawing speed. Then the level of the profile forming body 104 is varied suitably by axial displacement of the rod 132 until suitable parameters for the position of the profile forming body 104 relative to the shaping body 103, for the temperature and for the drawing speed have been found to thereby form a glass tube 105 having a predetermined inner profile. By means of the temperature, the drawing speed, the throughput and the position of the shaping body 103 in particular the wall thickness of the glass tube 105 can be varied.
For placing the profile forming body in this process firstly the flow of the glass melt needs to be stopped by means of the shaping body. Often this results in jamming of the outlet orifice 124 with glass and hence later in failures and/or batches of lower quality caused by glass dripping down along the outer surface of the glass tube. Furthermore, the above procedure for replacement of the profile forming body also bears a substantial accident hazard because the apparatus needs to be handled in the region near the outlet orifice 112 where there always exists the risk of a sudden discharge of glass melt.